A conventional step in the manufacture of a printed circuit board is optically imaging a pattern representative of the circuit to be printed onto the surface of the board, which has been coated with a photosensitive material. The photosensitive material is then processed to produce conductive circuit traces forming the circuit. The imaging process may be a contact imaging process, in which light from a powerful illumination source is directed to shine through a glass mask having a pattern defining the image to be formed on the substrate. This pattern is held close to the substrate surface being imaged. Step-and-repeat contact imaging is used to produce substrates having a number of identical circuits, each of which is much smaller than the substrate, with imaging and subsequent chemical processing occurring before the substrate is cut into individual segments, by sequentially illuminating the substrate through a mask which is stepped through an array of imaging positions. The imaging process may alternately be a projection imaging process, in which light transmitted through the mask including the pattern to be imaged is focused on the substrate through a projection lens system. A number of U.S. patents describe apparatus for this process of imaging a pattern without achieving capabilities to image both sides of a flexible substrate in a high-speed, flexible process forming high-density circuit patterns, which are of particular interest today.
For example, U.S. Pat. No. 5,548,372 to Schroeder et al describes contact imaging apparatus designed to provide accurately aligned printed circuits on both major sides of a printed circuit board layer. The apparatus includes patterns formed on glass masks attached to frames incorporating alignment pins and slots. The patterns include registration marks for alignment during the manufacture of the apparatus. During use, the apparatus allows accurate alignment of patterns on both sides of a PCB (printed circuit board) layer. Also disclosed is the apparatus with buttons used to pattern PCB layers having pre-drilled Z-axis holes. Thus, while this apparatus provides a method for aligning a mask on each side of the PCB board, the reliance on pins and holes for alignment is detrimental to using the apparatus in an automated imaging tool to produce PCB boards having the accuracy required for the presently significant high-density circuits. Thus, what is needed is tooling apparatus producing accurate alignment of the various element without using pins in holes, together with methods for handling flexible substrates, for changing the masks to produce different types of circuits, and for step and repeat imaging.
U.S. Pat. Nos. 4,924,257, 5,285,236, and 5,652,645, each to Jain, describe large image projection systems having large area exposure capability, in which an image from a moving mask is focused onto a substrate moving in a complementary fashion. Both the mask and the substrate move either in a scanning direction producing the image as an elongated strip, or in a lateral direction perpendicular to the scanning direction, so that the next strip of image can be produced with a subsequent scanning motion. A light source illuminating the mask is arranged to produce a light pattern having a hexagonal shape. Adjacent image strips overlap one another, with overlapping areas being illuminated by triangular portions of the hexagonal pattern, so that a uniform level of illumination is achieved. In the device U.S. Pat. No. 4,924,257, the mask and substrate move in opposite directions on opposite sides of the focusing lens, which is positioned to magnify the image from the mask. The mask is moved at a velocity greater than that of the substrate. In the devices of U.S. Pat. Nos. 5,285,236 and 5,652,645, the mask and substrate are both placed on an upper surface of a movable stage, with the mask being illuminated from below, and with the image being reflected from a steering mirror and transmitted through an objective lens providing a unity magnification and a reversing means rendering the image in the same orientation as the object. In the device of U.S. Pat. No. 5,285,236 are each essentially the same size. In the device of U.S. Pat. No. 5,652,645, the substrate is an elongated flexible strip, on which a number of images of the mask are placed. Provisions are made to move the strip in both longitudinal and transverse directions, as required by the imaging process.
U.S. Pat. No. 5,298,939 to Swanson et al. describes a scanning projection system for transferring an image from a stationary mask onto a stationary substrate by means of a moving optical system including a light source and a number of prisms, lens elements, and mirrors.
U.S. Pat. Nos. 4,667,301 to Tanimoto et al and 4,742,376 to Phillips describe step-and-repeat alignment and projection exposure systems for printing an image of a mask onto a semiconductive wafer. The system of U.S. Pat. No. 4,667,301 has a projection optical system for projecting a pattern image on a mask onto a substrate, a detector for detecting a two-dimensional misalignment of a projected pattern image and the substrate, and means for moving the substrate along orthogonal x- and y-axis directions and for rotating the substrate along a rotational direction within a plane defined by the x- and y-axis directions to eliminate the misalignment. The system of U.S. Pat. No. 4,742,376 uses a catadioptric lens having a unity magnification, which views an image of a mask through a prism, and which exposes a portion of the wafer through a beamsplitter. For alignment, stage reference means image a stage reference mark into the image plane of the lens when the stage reference means are illuminated by the projected mask image. A six-degree of freedom support system is controlled by focusing and leveling means to provide vertical position, pitch, and roll movement of the wafer being imaged.